Cabling configurations for hollow core optical fibers

ABSTRACT

A hollow core optical fiber and cable combination is configured to exhibit minimal SNR and loss degradation. This is achieved by either: (1) reducing the coupling between the fundamental and other (unwanted) modes propagating within the hollow core fiber; or (2) increasing the propagation loss along the alternative. The first approach may be achieved by designing the cable to minimize perturbations and/or designing the hollow core fiber to fully separate the fundamental mode from the unwanted modes so as to reduce coupling into the unwanted modes. Whether through fiber design or cable design, the amount of light coupled into unwanted modes is reduced to acceptable levels. The second approach may be realized through either fiber design and/or cable design to suppress the light in unwanted modes so that an acceptably low level of light is coupled back into the fundamental mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of pending U.S. patentapplication Ser. No. 16/475,531, filed Jul. 2, 2019, which claims thebenefit of PCT/US18/26713, filed Apr. 9, 2018, which claims the benefitof U.S. provisional patent application Ser. No. 62/482,900, filed Apr.7, 2017, all of which are incorporated herein by reference in theirentireties.

TECHNICAL FIELD

The present invention relates to hollow core optical fibers and, moreparticularly, to specific combinations and configurations of hollow coreoptical fibers and the associated outer cable structure best-suited forparticular applications.

BACKGROUND OF THE INVENTION

Hollow core optical fiber is a powerful technology platform offeringbreakthrough performance improvements in sensing, communications,higher-power optical pulse delivery, and the like. Indeed, inasmuch asits latency is almost equal to the propagation of an optical wave in avacuum, the hollow core optical fiber offers an attractive solution fordata centers, high-frequency stock trading communication links,distributed computing environments, high performance computing, etc. Inthe stock trading application, for example, the hollow core opticalfiber is contemplated as allowing for decreased data transmission timesbetween trading computers, enabling trading programs to completeprogrammed trading transactions more quickly.

While there has been much progress on developing hollow core fiberscapable of meeting stringent low latency requirements, the fielddeployment of such fibers has been found to be compromised by multipathinterference (MPI), which is caused by the fact that hollow core fiberis intrinsically multimode. Moreover, since the index of refraction ofthe higher-order modes (HOMs) and surface modes are significantlydifferent that the index of refraction of air, these HOMs are affecteddifferently by perturbations within the fiber (and are therefore asource of MPI). The presence of MPI results in power fluctuations thatrender the transmitted signal unrecoverable. Indeed, it is been foundthat the act of “cabling” such fibers (“cabling” referring to thefabrication process of covering the fiber (or collection of fibers)within multiple layers of protective material) introduces (at times)catastrophic degradation. It appears that one or more failure mechanismsis related to perturbations impressed on the fiber by the cablestructure itself.

Thus, a need remains in the art for providing a combination of hollowcore optical and suitable cable structure that does not degrade signaltransmission due to signal interference effects. More particularly, aneed remains to provide a hollow core optical fiber cable assembly thatminimizes the presence of MPI along the individual hollow core opticalfibers.

SUMMARY OF THE INVENTION

The present invention addresses the needs in the art and is directed toascertaining specific combinations and configurations of hollow coreoptical fibers and outer cable structures that are best-suited forparticular applications.

In accordance with one or more embodiments of the present invention, anoptical cable comprising one or more individual hollow core opticalfibers is specifically designed with an understanding of the sources ofMPI for a specific hollow core fiber design, and then designing a cablestructure that either reduces the impact of mode coupling (for example,by increasing HOM loss without unduly impacting fundamental mode loss)or, alternatively, does not introduce additional mode mixing in fiberconfigurations that have been designed to exhibit minimal (acceptable)mode mixing.

Various embodiments of the present invention may utilize different typesof hollow core optical fiber, including high-birefringent hollow coreoptical fiber, higher-order-mode (HOM) suppressing hollow core opticalfiber, or various combinations of these types of fibers that are capableof minimizing unwanted modes (including HOMs, polarization modes,surface modes, and the like). These various embodiments are then pairedwith a selected cable design that either further reduces mode mixing (orthe impact of mode mixing) or does not introduce additional mixing. Insome embodiments, selected cables may be configured to minimizeperturbations present along the cabled fiber. In other embodiments,selected cables may be configured to introduce a predetermined level ofperturbation (e.g., using bend-induced perturbations) to preferentiallyattenuate unwanted modes, thus reducing MPI. The specific cable designsare selected from, for example, slotted-core cables, loose buffercables, tight buffer cables, and loose-tight buffer cables. Specificparameters of each of these types of cables are particularly configuredfor a given application to address problems associated with mode mixing.For example, fiber bend along a stranded cable is a parameter thataffects the mode mixing within the fiber (bend-induced perturbations)and is defined by pitch length, pitch radius, and pitch angle of thefibers within the cable configuration.

Indeed, an exemplary embodiment of the present invention takes the formof a method of configuring an optical fiber cable assembly including atleast one hollow core optical fiber. This exemplary embodiment comprisesthe following: determining a maximum allowable MPI level permitted in afinal cable assembly and selecting a hollow core fiber and cableconfiguration suitable for maintaining MPI below the determined maximumallowable level. If the selected hollow core fiber configuration issensitive to mode mixing and is therefore not likely to exhibit MPIwithin acceptable limits, the method includes selecting a cable designthat intentionally introduces perturbations sufficient to reduce MPI. Ifthe MPI of the fiber is within acceptable limits, then the methodperforms the step of selecting a cable design that maintains MPI belowthe determined maximum allowable level. This includes selecting a cabledesign that reduces perturbations on the fiber to inhibit mode mixing.

Other and further embodiments and aspects of the present invention willbecome apparent during the course of the following discussion and byreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings:

FIG. 1 is a diagram depicting the evolution of multipath interference(MPI) along a hollow core optical transmission fiber;

FIG. 2 is a schematic view of an exemplary hollow core optical fiber;

FIG. 3 is a schematic of an alternative hollow core optical fiber,configured as a polarization selective hollow core fiber;

FIG. 4 is a schematic of an exemplary mode-suppressing hollow coreoptical fiber, in this case including a pair of shunt cores;

FIG. 5 is a schematic of an alternative configuration of amode-suppressing hollow core optical fiber, in this case including a setof six shunt cores;

FIG. 6 depicts an exemplary cross-section view of a set of hollow coreoptical fibers within a cable structure, in this case using a loose tubebuffer to surround each fiber;

FIG. 7 is a cross-section view of an alternative cable structure, inthis case a slotted core cable;

FIG. 8 is an isometric view of an exemplary slotted core for use in thestructure of FIG. 7;

FIG. 9 is a simplified line diagram illustrating the parameters of pitchlength, radius and angle, used in determining an amount of bend-inducedperturbation introduced into a hollow core optical fiber by thesurrounding cable structure;

FIG. 10 is a transmission spectrum showing that cross-coupling of modesresults in large variation with wavelength of the detected signal;

FIG. 11 shows the bit error rate (BER) of a signal transmitted throughthe cabled fiber of FIG. 10;

FIG. 12 illustrates an improvement of the linearity in the transmissionspectrum for cabled fiber vs. spooled fiber for an alternative fiberconfiguration;

FIG. 13 illustrates how the cabling in this particular configuration ofFIG. 12 improves the BER, to a value very similar to the BTBconfiguration;

FIG. 14 shows the peak-to-peak transmission variation as a function ofwavelength of fiber; and

FIG. 15 is a flowchart illustrating a set of steps that may be followedin determining an optimum combination of hollow core optical fiber andcable structure in accordance with the present invention.

DETAILED DESCRIPTION

As will be discussed in detail below, the present invention relates toassessing the properties of various types of hollow core optical fibersand cabling structures, and determining an optimum combination of fibertype and cable structure that is particularly well-suited for a givenapplication. For example, the transmission of optical signal light alongan “air” core (as is the case for various configurations of hollow corefiber) provides for transmission speeds that are 30% greater than thatassociated with standard silica core optical fibers. As mentioned above,this feature has particular applications to high-frequency tradingcompanies, which rely on low latency communication links. Low latencyalso has applications in datacenter/supercomputer applications, wherehundreds of kilometers of optical cables are used to interconnectthousands of servers.

The specific structure of the cable used to encase these hollow corefibers has been found to impact their performance, at times to the pointwhere a specific data transmission rate specification cannot be met. Onegoal of the present invention is to determine a proper hollow core fiberand cable combination that minimizes signal impairments such that thefinal cable assembly is acceptable for use in low latency applications.Inasmuch as a given hollow core fiber may exceed a given specificationin its “bare fiber” form, but then fail the same test when packagedwithin a cable, it is important for the combination of fiber and cablestructure to be better understood and designed as a symbiotic component.

It is believed that a significant source of poor performance of cabledhollow core fibers resides from cable-created perturbations that areimpressed on the fiber and introduce mode mixing. The present inventionaddresses this problem, offering solutions that include either carefullydesigning a hollow core fiber that does not support the propagation ofmultiple modes, more specifically rapidly attenuates unwanted modes asthey propagate along the fiber (perhaps with a specific cable structurethat ensures the maintenance of the singlemodedness), or utilizing ahollow core fiber that may not have high attenuation of unwanted modes(e.g., higher-order modes (HOMs), polarization-dependent modes, orsurface modes) and creating a cable structure that is able to reducethem to acceptable levels. In the case of fibers that are designed tohave rapid attenuation of unwanted modes, the cable should be designedto minimize mode mixing (i.e., control perturbations).

More particularly, a source of problems in cabled hollow core fibersresults from MPI, as mentioned above. FIG. 1 depicts an exemplary hollowcore optical fiber transmission line experiencing MPI. As shown, MPIoriginates when an optical signal propagating in the fundamental (LP₀₁)mode couples into other fiber modes (e.g., HOMs, surface modes), shownin positions A, B and C in FIG. 1. At a later point along thetransmission line, these “other-moded” signals may then couple back intothe fundamental mode (where random phasing occurs). This process repeatsall along the transmission line to an endpoint termination of the fiber.It is important to note that when the diverted signal is coupled back tothe fundamental mode signal, it is impossible to separate them andremove the unwanted signal copy.

When a portion of the propagating signal is coupled into one of theunwanted modes, its phase and/or amplitude may be delayed (relative tothe original signal) if there is a difference in the phase and/or groupvelocity of the different modes. This delay results in interferencebetween the signal and the unwanted signal copy. Additionally, theunwanted mode(s) signal can be preferentially attenuated if the lossexperienced by the signal propagating in the fundamental modes and thesignals propagating in the unwanted modes are different. Inasmuch as thesignal that propagates through all of the alternative modes has randomattenuation delay and phase shift, and since there are many alternativemodes and re-coupling events during signal transmission, it can beconsidered that the signal reaching the far-end termination of thetransmission line exhibits added “noise”, not deterministic distortion.Intensity noise caused by distributed multipath interference ofdifferent modes is described by S. Ramachandran et al. in the paperentitled “Measurement of multipath interference in the coherentcrosstalk region”, published in IEEE Photonics Technology Letters, Vol.15, No. 8, August 2003. As indicated in that work, the presence of MPIcan be detected by measuring the attenuation spectrum of the fiber witha narrow linewidth swept-wavelength source.

The presence of MPI, therefore, affects both the reach and the bandwidthof the cabled hollow core transmission fiber. As is well known in theart, the acceptable level of MPI to provide a minimum bit error rate ina transmission link depends on many factors, including bit rate, signalformat, the nature of the transceiver, and the desired system margin.

Thus, the problems related to MPI in a hollow core fiber can bedescribed in terms of signal-to-noise ratio (SNR) evolution and bedivided into three stages: (1) coupling into the alternative mode(s),shown as points A, B, and C in FIG. 1; (2) propagating along thealternative mode(s), shown as lengths a, b, and c in FIG. 1; and (3)re-coupling back into the original path of the fundamental mode signal,shown as points A′, B′, and C′ in FIG. 1. In terms of SNR degradation,the initial coupling into the unwanted mode(s) reduces the remainingsignal power propagating along the main signal path in the fundamentalmode. The following propagation of the unwanted mode(s) along thealternative paths does not directly impact SNR. Lastly, there-introducing of the signals propagating along alternative paths intothe main signal path of the fundamental mode causes coherent cross-talkamong co-propagating modes that increases the noise, as mentioned above,and is considered as the main source of SNR degradation (since itdirectly transfers the signal power to the noise power).

It is also to be noted that the coupling mechanism is reciprocal. Thatis, when there is a condition to couple from the fundamental mode toother modes (e.g., phase matching) there also exists a condition tocouple the light back from other modes (i.e., the first and third stagesalways happen simultaneously).

Hollow core optical fiber experiences all of these problems in ways thatare very different than in conventional silica core fibers. In hollowcore optical fiber, the index difference between the fundamental modeand other unwanted modes (both HOMs and surface modes) is much higherthan that of conventional silica fibers. As a result, there may be moreof an environmentally-dependent phase shift in hollow core opticalfibers, necessitating higher attenuation of unwanted modes than inconventional solid core fiber.

In light of all of this, a goal of the present invention is to design afiber and cable combination with minimal SNR and loss degradation. Thiscan be achieved, as will be described in detail below, by either: (1)reducing the coupling between the fundamental and other modes (i.e.,reduction of the first and third stages of MPI, as defined above); or(2) increasing the propagation loss along the alternative paths (i.e.,during the second stage, as defined above). The first approach can beachieved, in accordance with the principles of the present invention, bydesigning the cable to minimize perturbations and/or designing thehollow core fiber to fully separate the fundamental mode from theunwanted modes so as to reduce coupling into the unwanted modes. Whetherthrough fiber design or cable design, the amount of light coupled intounwanted modes is reduced to acceptable levels. The second approach canbe realized through either fiber design and cable design to suppress thelight in unwanted modes so that an acceptably low level of light iscoupled back into the fundamental mode. Referring now to FIG. 2, thereit shows a schematic cross section view depicting basic features of atypical prior art hollow core fiber 10. In particular, the hollow corefiber shown in FIG. 2 comprises a cellular structure where an individualcell C includes a cellular space S and a cell wall W enclosing thecellular space. The individual cells are of substantially similarnominal dimensions, cell wall thickness and chemical composition. Thecells in this example are shown to be in a honeycomb pattern; however,cells having other geometrical shapes and pattern are not precluded.Adjacent cells are generally connected at the cell walls and distributedin a lattice pattern (also referred to at times as a cellular lattice ora web of cells) having a substantially uniform lattice spacing(substantially similar nominal spacing between the centers of adjacentcells). The lattice pattern in general, is uniform and regular. For easeof discussion, the cells comprising the cellular lattice will bereferred as regular cells hereinafter.

The cellular lattice functions as an inner cladding extending up to apre-determined radial distance and the outer edge of the cladding issurrounded or enclosed in another outer cladding (not shown). Acontiguous group of intentionally omitted regular cells arranged in apredetermined regular pattern in the cellular lattice, comprise a hollowcore 12. The size and shape of hollow core 12 is determined respectivelyby the number and arrangement of omitted cells. The dimension and shapeof the hollow core is often described in terms of the number of missingcells (13 cell, 17 cell, 19 cell, 33 cell, and so forth). While thehollow core in this example is approximately a hexagon placed at thecenter of the cellular lattice, different shapes and/or placement is notprecluded. For example, a hollow core may be circular and be placedsymmetrically or asymmetrically within the cellular lattice structure.The light to be transmitted in the hollow core is confined and guided inthe hollow core by the photonic bandgap resulting from the periodicstructure of the cellular matrix outside the hollow core (a conceptsimilar to electronic band gap in a semiconductor crystal).

The hollow core functions as a transmission waveguide supportingpredominantly a single, preferably a fundamental mode, but other modesincluding one or more HOMs that may be a signal mode or an unwanted modemay also be present in the hollow core. In the example of a hollow corefiber shown in FIG. 2, hollow core 12 is disposed to exhibit reflectionsymmetry along one or both orthogonal axes passing through the core.However, hollow cores having different reflection symmetries have beenused to selectively impart a desired transmission characteristic forexample, a preferred polarization mode, selective loss of an undesiredcore mode such as a higher order mode (HOM), etc.

In particular, FIG. 3 shows a schematic cross section of a polarizationselective hollow core optical fiber 20. Similar to fiber 10 of FIG. 2,an inner cladding comprises a cellular lattice. More specifically, thecellular lattice includes cells having a cellular space S and a cellwall W. The cells are connected at the cell walls and distributed in alattice pattern similar to what is described in reference with FIG. 2.The cellular lattice has a regular pattern of cells that may approximatea triangular lattice, rectangular lattice, honeycomb lattice, or otherwell-known lattice patterns. A contiguous group of intentionally omittedregular cells arranged in a predetermined regular pattern in thecellular lattice, comprise a hollow core 22. Omission of cells in thehollow core region locally disrupts the continuity of the latticestructure only in the core region while retaining the lattice structurearound the core. The pattern of cells around the core region maycorrespond to an ideal (e.g., triangular) pattern, or have a distortionpattern (e.g., triangular lattice distorted by enlargement of the core).

In the case of polarization selective hollow core optical fiber, 20 apredetermined set of the regular cells in the cellular lattice aresubstituted with a corresponding set of a different type of cell(s) 24shown shaded in this view (only one labeled for clarity). Thesubstituted cells (also referred to as “leakage cells”) in general,replace a regular cell at a nominal lattice site. The leakage cellsdiffer from the regular cells in at least one physical and/or chemicalaspect. For example, in the embodiment shown in FIG. 3, the leakagecells 24 are of a different shape and size. More specifically, leakagecells in this group are collapsed or compressed in the verticaldirection (referring to the figure) relative to nominal dimension of theregular cells. Alternatively, the leakage cells may be generated byexpanding a regular cell from their nominal dimension. It should beunderstood that leakage cells do not disrupt the lattice pattern ingeneral but it may cause a distortion in the lattice pattern,particularly if the leakage cells substantially differ in physicaldimensions from the regular lattice cells.

In a variant aspect, leakage cells are often placed along a line segmentbetween the core-cladding interface and the outer edge of the cladding.For example, the leakage cells may be centered on the line segment, orthey may just touch the line segment (on one or both sides). The leakagecells may be disposed along one symmetry axis of the fiber as shown inthis example (FIG. 3). However, other distributions or patterns may beused to achieve desired transmission characteristics such as,polarization state, selective suppression of a mode, HOM suppression toname a few.

The leakage cells locally modify the properties of the cladding layer.Leakage cells are designed to collectively provide a leakage path in thecladding layer. A leakage path may extend across the cladding from thecore-cladding interface towards the outer cladding as shown in FIG. 3.The fiber appears to be ‘zipped’ together, along a leakage path. Morespecifically, one or more leakage cells positioned close enough to eachother in the cladding region provide one or more continuous leakagepaths for some of the core modes by selectively coupling them to thecladding and provide optical coupling of selected core modes to theouter cladding boundary. Although FIG. 3 shows one type of a leakagepath other possible options are not precluded. In any of thesevariations, a cladding structure that does not impart unwantedperturbations onto the created leakage paths is preferred (for example,a slotted core cable configuration or a loose tube buffer).

Beyond the basic hollow core optical fiber structures shown in FIGS. 2and 3, there has been an increasing effort in developing configurationsthat suppress unwanted higher order modes. In particular, various typesof hollow core fibers have been developed that incorporate additionalhollow regions (often referred to as “shunt cores”, or simply “shunts”)that surround the core and function to out-couple HOMs from the centralcore region. The basic design of this configuration is shown in FIG. 4,where an HOM-suppressing hollow core fiber 40 has a cellular matrixstructure of holes (voids) in an interconnected web of silica glass.Similar to the configurations shown in FIGS. 2 and 3, individual cells(matrix cells) in the web connect with neighboring cells at the cellwalls (boundaries) that in general have uniform nominal thicknessthroughout the web. In a typical hollow core fiber, the cells aredistributed in a regular periodic arrangement (in two dimensionalphysical space cross-section perpendicular to the axis of the fiber)with a uniform nominal spacing between the holes (substantially similarto a lattice spacing in a crystal). The cellular lattice has a regularpattern of cells that may approximate a triangular lattice, rectangularlattice, honeycomb lattice, or other well-known lattice patterns.

As with the configurations describe above, a central contiguous regionof fiber 40 is devoid of the cellular matrix structure, defining ahollow core 42. Hollow core 42 extends along the length of fiber 40. Thesize (diameter) of hollow core 42 is determined by the number of cells(for example a 19-cell or a 7-cell core, etc.) that are contiguouslymissing in the core region. Specific geometry of the core (for example,elliptical, oblong, circular or hexagonal core shape) may be determinedaccording to the required polarization and transmission propertiesdesired for a particular application. Besides hollow core 42,mode-suppressing hollow core optical fiber 40 includes one or morehollow regions 44 disposed to surround hollow core 42 and also extendalong the length of the fiber. The additional one or more hollow regionsare in general smaller in size as compared to the core and oftenreferred as shunt core(s) or simply shunts in this context. Theconfiguration of FIG. 4 is at times referred to as a “tri-core”, basedon the arrangement of central hollow core region 42 and a pair of shunts44-1 and 44-2. FIG. 5 illustrates an alternative configuration (referredto at times as a “hepta-core fiber”), where here a hollow core opticalfiber 50 includes a central hollow core region 52 and a plurality of sixseparate shunts (labelled as 54-1, 54-2, . . . , 54-6) disposed tosurround central hollow core 52. Other configurations are possible. Thedesired level of attenuation of the HOMs depends on the amount of lightcoupling such that the cross-coupled light is preferably less than −15dB, and more preferably less than −30 dB, that of the signal. In typicalhollow core fibers, this can be achieved with HOM attenuation of morethan 7.5 dB/km, or more preferably more than 1 dB/m. In some instances,HOM attenuation of more than 40 dB/m is required.

MPI may also be caused by coupling to and from well-known surface modes;that is, light that propagates predominant within the glass regionssurrounding the core. The nature of surface mode coupling is similar toHOM and polarization coupling. Because the modes have differentpropagation constants, a perturbation is required to provide phasematching to induce coupling, with the strength of coupling depending onthe coupling coefficient and the perturbation magnitude. In the case ofsurface modes, the spatial period of the perturbation is similar to thatwhich causes microbending in the conventional fiber. For many relevantmodes which cause MPI, the perturbation length scale ranges fromhundreds of microns, to millimeters, to centimeters. To maintainacceptable levels of MPI, surface mode attenuation should be greaterthan 0.1 dB/m, though in come cases, attenuation should exceed 0.3 dB/m.

It may be that such a fiber is sensitive to mode mixing; that is,various types of slight perturbations to the fiber (temperaturevariation, bending, microbending, spatially-varying strain, etc.) aresufficient to disturb the fiber's geometry such that the unwanted modesre-appear and increase MPI. If this is a possibility, embodiments of thepresent invention allow the utilization of a cable configuration (forexample, a slotted cable) that imparts little or no perturbations to thefiber. Depending on the degree of sensitivity, other cableconfigurations (such as loose buffer or loose-tight buffer) may besufficient. Various ones of these fiber and cable combinations arediscussed in detail below.

Further, it may be that such a fiber is sensitive to mode mixing; thatis, various types of slight perturbations to the fiber (e.g.,temperature variation, microbending, bending, etc.) are sufficient todisturb the fiber's geometry such that the unwanted modes re-appear. Ifthis is a possibility, embodiments of the present invention allow theutilization of a cable configuration that imparts a controlled level ofperturbations to the fiber to attenuate the unwanted modes, reducingMPI. Various ones of these fiber and cable combinations are discussed indetail below.

Alternatively, it may be the case that such a fiber that is configuredto eliminate unwanted modes is insensitive to mode mixing. For example,a high birefringent hollow core fiber (such as shown in FIG. 3) is nottypically affected by perturbations and maintains its single mode (andsingle polarization) operation. For these situations, the specificstructure of the cable itself is not as critical and various types ofcabling (including tight buffer) may be used. It is to be noted,however, that if suppression of HOMs also results in increasing the lossof the propagating fundamental mode to an undesirable level, a cableconfiguration that intentionally introduces perturbations to the hollowcore fiber may be a preferred alternative (for example, a cableconfiguration providing bend-induce perturbations).

In accordance with one or more other, alternative embodiments of thepresent invention, a hollow core optical fiber is configured toattenuate unwanted modes (instead of attempting to eliminate thecoupling to unwanted modes), thus reducing the requirements on the cablestructure to assist in minimizing mode mixing. In particular, one ormore specific configurations of these alternative embodiments consistsof using a hollow core fiber including one or more shunt cores (smallerin diameter than the main core) disposed around the core, such asdepicted in FIGS. 4 and 5. As mentioned above, one needs to carefullyconfigure these fibers including shunt cores to ensure that loss in thedesired, fundamental mode is not significant. This fiber design furtherincludes bend-induced perturbations to increase loss in the unwantedmodes. In these cases, it is preferred to utilize a cable structure thatincreases HOM loss.

As mentioned above, the amount of perturbation experienced by a cabledhollow core optical fiber may be controlled by the configuration of theinduced stress on the fiber itself in the final cable construction.Control of the induced stress is, in turn, achieved primarily throughthe design of the cable itself. For example, the hollow core fiber maybe contained within a loose tube buffer cable construction, or a tightbuffer cable construction, or even some combination of the two (referredto at times as a loose-tight buffer). One example cable design,specifically configured to minimize the possibility of perturbations onthe hollow core fiber is the “slotted core” cable. Indeed, a loose tubebuffer or slotted core cable will produce little or no perturbationwhile a loose-tight or tight buffer is known to produce higher amountsof perturbation on the hollow core fiber within the finished cable.

FIG. 6 is a cross-section illustration of an exemplary cable 70containing a set of four hollow core fibers 72 (specifically illustratedas 72-1, 72-2, 72-3 and 72-4). Each fiber 72 is surrounded by a bufferlayer 74, which is applied over the finished (coated) fiber. For a“tight buffer” configuration, the material forming buffer layer 74 istightly applied, leaving no air space between the coating on the fiberand the inner diameter of the buffer. The specific “tightness” of buffer74 over fiber 72 can be controlled by the buffer material selection andthe manufacturing process used to apply the “tight buffer” over thecoated fiber.

A “loose tube” buffer, on the other hand, is achieved by allowing an airgap, by design and manufacturing process, to exist between the coatedfiber and the ID of the buffer. The specific illustration of FIG. 6shows such a gap “g” between hollow core fiber 72-1 and its associatedbuffer 74-1. This air gap may be filled with a water-blocking gel orswelling compound to prevent ingress of moisture or water, which isoften required for outdoor installations of hollow core fiber cable. A“loose-tight” buffer is considered to be a specific type of “loose tube”buffer, where the amount of gap is somewhat reduced, thereby controllingthe amount of perturbation on the hollow core fiber.

FIG. 7 depicts a cross-section of a slotted core cable, useful forapplications of the present invention where it is desired to minimizeperturbations felt by the hollow core fiber itself. FIG. 8 is anisometric view of a slotted core cable. Slotted core 80 may beconfigured to comprise multiple slots 82, with a separate hollow corefiber 84 disposed in each slot. For higher fiber count cables, more thanone individual fiber may be disposed in each slot. As will be discussedin detail below, this cable configuration allows for the minimal amountof perturbation on the hollow core fiber. As with the loose tube cableconstruction, the free space within the slotted cable may be filled withwater blocking gel or swelling compound when the finished cable isintended for use in an outdoor (or “wet” indoor) installation.

FIG. 9 is a simple line drawing showing how fiber bend can be designedinto a stranded cable to control bend-induced perturbation of the hollowcore fibers within the cable. Here, a single hollow core optical fiber100 is shown as stranded (wrapped) a single time around a cylinder 110.The fiber itself may be contained within a loose tube buffer, tightbuffer, or slotted core. Cylinder 110 may be a central strength memberof a required diameter to provide the necessary mechanical strength tothe finished cable construction, with the required spacing betweenmultiple, buffered hollow core optical fibers stranded (or wrapped)around cylinder 110. Cylinder 110 typically comprises a compositematerial of epoxy-glass or aramid-glass, or a solid steel wire.

As mentioned above, one or more embodiments of the present invention mayutilize a specific bend-induced perturbation of the hollow core opticalfibers, provided by the cable structure, to control the amount of modemixing present in the final assembly such that the power level of thefundamental mode does not drop below an unacceptable level. Thebend-induced perturbation of a hollow core fiber is achieved bycontrolling the pitch length ρ, pitch radius r, and pitch angle ϕ of thefibers within the cable configuration. The pitch angle ϕ in a givencable configuration is expressed as:

${\sin \varphi} = {\frac{2\pi \; r}{\sqrt{\left( {2\pi \; r} \right)^{2} + \rho^{2}}}.}$

Further, the bend radius α and the radius of curvature β of a hollowcore optical fiber is indicated as follows:

$\alpha = {\frac{1}{\beta} = {\frac{r}{\sin^{2}\varphi}.}}$

For a given cable configuration with a pitch radius r, the perturbationon an included hollow core fiber can be increased by decreasing thepitch length ρ, which thereby increases the pitch angle ϕ. When thepitch angle ϕ is increased, the bend radius α of the fiber decreased,thereby increasing the radius of curvature β of the hollow core fiberwithin the final cable structure. Bend-induced perturbations on a hollowcore fiber may be decreased by increasing the pitch length ρ for a givencable construction.

The impact of MPI and cabling is illustrated in FIGS. 10-14. FIG. 10 isa transmission spectrum made using a swept-wavelength source. Asdescribed above, cross-coupling of modes results in large variation withwavelength of the detected signal. In this example, bare fiber (curveBF) shows very little variation as a function of wavelength, while thecabled fiber shows significant reduction in overall transmission andlarge wavelength variation. FIG. 11 shows the bit error rate (BER) of asignal transmitted through the cabled fiber. At a wavelength of 1530 nmwhere transmission has relatively low variation (i.e., a few dB), BER issimilar to the back-to-back (BTB) value while wavelength variation ofmore than about 5 dB causes significant BER penalty. In this example,while the bare fiber performs well, cabling has induced excessiveperturbation that causes MPI and degrades transmission.

FIGS. 12 and 13 show an alternative example that demonstrates thatproper cabling can improve that transmission characteristics compared toa spooled fiber. In this example, the fiber is sensitive topolarization-dependent effects (differential group delay—DGD) and isperturbed by overwinds and marobending on the spool but proper cablingeliminates unwanted polarization effects. In particular, FIG. 12illustrates an improvement of the linearity in the transmission spectrumfor the cabled fiber vs. the spooled fiber. FIG. 13 illustrates how thecabling in this particular configuration improves the BER, to a valuevery similar to the BTB configuration.

FIG. 14 shows the peak-to-peak transmission variation as a function ofwavelength of fiber that is relatively robust to perturbations and hasacceptable performance when spooled bare with both overwinding andmacrobending. It is clear that the performance is essentially unchangedafter cabling. Peak-to-peak variation of about 10%, and in some cases upto 20%, is acceptable for signal transmission links.

As mentioned above, an aspect of the present invention relates toproviding an optical cable comprising one or more individual hollow coreoptical fibers that is specifically configured with an understanding ofthe sources of MPI for a specific hollow core fiber design, and thendesigning a cable structure that either compensates for an unacceptableamount of mode mixing in the fiber configuration or, alternatively, doesnot introduce additional mode mixing in fiber configurations that havebeen designed to exhibit minimal (acceptable) mode mixing.

FIG. 15 is a flowchart outlining an exemplary set of steps that may befollowed in selecting a specific hollow core optical fiber configurationand cable structure that meets these requirements. The process begins atstep 200 by defining a set of system requirements necessary for aspecific application. For example, a certain level of MPI may be one ofthe driving design parameters. Once this is established, the next stepin the process is to determine the proper type of hollow core opticalfiber that is best-suited for this application and the systemrequirements (step 210). In one particular methodology, a decision maybe made at this point whether to select a hollow core fiber design thatremains multi-moded, but is defined to eliminate coupling to unwantedmodes (shown as element 220 in the flowchart), or a hollow core fiberthat is formed to be mode-suppressing, such as by including shunt cores(shown as element 230 in the flowchart).

Following the path from element 220, the selected fiber is evaluated todetermine its sensitivity to mode mixing (decision point 240). If thefiber is sensitive to mode mixing, a cable structure is then selected(step 250) that imparts little or no perturbations on the fiber (e.g.,slotted core cable, loose buffer cable). Returning to decision point240, if the fiber is insensitive to mode mixing, a tight buffer cablemay be selected (step 260).

Returning to element 230 in the flowchart, once the decision has beenmade to use a mode-suppressing hollow core fiber design, the next stepin the design process is to ascertain how much loss is found in theunwanted modes (step 270), and to decide if the amount of loss isacceptable (decision point 280). If the loss is acceptable, any suitablecable structure may be employed (step 300). However, if the amount ofloss in the unwanted modes is too low, a cable structure cable ofintroducing perturbations along the fiber is a proper selection (step290).

It is to be understood that the methodology outlined in FIG. 15 isexemplary only, and various other considerations may be added to thedecision-making process. For example, the selected combination of fiberand cable should not degrade other fiber parameters (i.e. should notintroduce a large amount of DGD or an excessive amount of fundamentalmode loss). Since high overall coupling may come from relatively lowlocal coupling monotonically distributed along the fiber length, and thecoupling process is typically reciprocal, the light will be coupledcontinuously between the fundamental mode and the unwanted modes. Thismay result in an MPI-like penalty, even in the central launch (i.e.,butt-coupled) case, or in the presence of a mode filter at the far-endtermination of the hollow core fiber cable. In this case, either themode coupling should be reduced to achieve the desired crosstalk level,or an additional unwanted mode loss mechanism should be introduced.

What is claimed is:
 1. A method of configuring an optical fiber cableassembly including at least one hollow core optical fiber having a firstmode and one or more unwanted modes, comprising: determining a minimumallowable higher order mode (HOM) signal propagation loss levelpermitted within the one or more unwanted modes in the hollow coreoptical in the final cable assembly; selecting a hollow core fiberconfiguration suitable for maintaining the HOM signal propagation losslevel above the determined minimum allowable level; if the selectedhollow core fiber configuration is sensitive to mode mixing, selecting acable design that intentionally introduces perturbations on the fibersufficient to reduce the HOM signal propagation loss level above theminimum allowable level in the presence of mode mixing sensitivities;otherwise, if the selected hollow core fiber is insensitive to modemixing, selecting a cable design that maintains the HOM signalpropagation loss level above the determined minimum allowable level.